The present invention relates generally to control valves and, more particularly to a micro control valve which utilizes mechanical beam buckling to open and close a valve orifice.
In many fluid flow control applications, especially in pneumatic flow control applications, it is desireable to use a control valve which may be actuated quickly and with relatively low energy consumption in response to an electronic control signal. It is also desireable in many applications that the control valve have a relatively small dead volume. Various applications for valves having such characteristics are described in U.S. Pat. Nos. 4,821,997 and 4,824,073 of Mark Zdeblick which are hereby specifically incorporated by reference for all that is disclosed therein.
U.S. Pat. No. 4,824,073 discloses that one known way of controlling the flow of a fluid using an electrical pulse is the electric to fluidic valve developed by Steve Terry of Stanford University. This valve uses a substrate such as silicon which has a thin membrane machined therein. A cavity is formed by the etching of a hole almost completely through the substrate. This leaves a thin bottom wall for the cavity which is used as a flexible membrane. Attached to the side of the first substrate in which the membrane is formed is a second substrate which has a manifold-type cavity etched therein with a passageway or nozzle formed in a wall of the manifold cavity for entering or escaping gas. The manifold cavity also has other ports formed therein to complete a fluid path into the manifold and out the nozzle or vice versa. The manifold cavity in the second substrate is positioned over the membrane of the first substrate such that when the manifold of the first substrate is flexed, it contacts a sealing ring formed around the nozzle of the manifold cavity thereby closing off the fluid flow path between the nozzle and the other ports into the manifold cavity. With the membrane of the first substrate in an unflexed position, the nozzle in the manifold cavity would not be pinched off, and fluid would be free to flow through the input port and the manifold cavity and out through the nozzle or vice versa. The membrane of the first substrate is forced to flex by mechanical forces exerted thereon by a piston. This piston is driven by a solenoid or other type of electromagnetic device.
One disadvantage of the above described valve configuration is that the solenoid requires a high-power source, and is a large power consumer. Further, the solenoid or other electromagnetic device is large and heavy. The cavities in the first and second substrates could be formed with much smaller dimensions if it were not for the fact that the solenoid is large. Because the first and second substrates are silicon wafers which are etched using conventional planar photolithography techniques, it would be possible to make the electric to fluidic valve much smaller in dimension were it not for the solenoid. Such a prior art electric to fluidic valve construction is inefficient in its use of space. Because the solenoid is mechanically attached to the first substrate such that the piston of the solenoid pushes against the membrane in the first substrate and because the solenoid is large enough to consume much of the wafer space, generally only three such valve structures can be formed on a single silicon wafer. Such a structure is relatively expensive to build, and the bond between the solenoid and the glass is difficult to make. Generally, the solenoid is attached to a thick pyrex wafer by nuts and bolts. This form of attachment is both expensive to fabricate and a major source of failures. Further, such a structure has a moving part which can be another source of failure. The principal defect of such a structure, however, is the fact that the entire structure cannot easily be mass-produced with planar lithography techniques. This is because the solenoid cannot be manufactured by such techniques.
U.S. Pat. No. 4,824,073 also discloses an electric to fluidic valve utilizing the principle of expansion and pressure rise of a fixed volume of gas or fluid when heated to deflect a flexible wall or thin membrane forming one or more walls of the cavity in which the gas or fluid is contained. The deflection of the membrane may be used to seal or unseal a fluid passageway from an input port, through a manifold cavity and out an output nozzle or vice versa. The valve may also be operated linearly to provide a linear range of fluid control, i.e., the valve may be controlled to modulate the rate of fluid flow through the valve in accordance with the magnitude of a control signal. The deflection of the membrane may also be used as a sensor indication for purposes of determining temperature changes or the magnitude of other phenomena to be measured.
The principal elements of each design will include a cavity formed in a substrate where one wall of the cavity is a thin, flexible membrane. The cavity encloses a fixed number of moles of gas or fluid, and there will be some method or means of raising the temperature of the fluid in the cavity so as to cause the vapor pressure to rise in the case of a fluid or to cause expansion and increased pressure in the case of a gas. This heating of the material in the cavity may be accomplished in any one of a number of ways. One way is the use of a resistive heating element on one wall of (in the case of diffused resistors, in one or more walls) or located somewhere inside the volume of the cavity such that electrical current may be passed through the resistive element to generate heat and heat the fluid trapped in the cavity.
A typical structure utilizes a silicon-pyrex sandwich for the membrane cavity and heating structure. The membrane chamber is formed in a silicon wafer by etching a trench in the wafer substantially completely through the silicon wafer but stopping short of the opposite side of the wafer by a margin which is equal to the desired thickness of the membrane of the membrane chamber. Other signal processing circuitry, such as power transistors or full feedback control systems with multiplexed input and output ports, may have been previously fabricated on the balance of the wafer in conventional processing. This circuitry can be used in conjunction with the electric to fluidic valve formed by the membrane chamber thereby forming a valve or transducer with its own interface circuitry located on the same silicon wafer as the valve itself using compatible processing steps. The same of course is true for sensor applications where the membrane chamber is used as a transducer. The signal processing or other circuitry built elsewhere on the wafer may then be used to signal process, condition or otherwise deal with the output signal from the transducer for their operation.
The surface of the silicon substrate having the membrane as part thereof is sandwiched with another wafer in which a manifold having an input port and a nozzle is formed (the nozzle may be the input port and the other port may be the output port also). The fluid manifold position is keyed so that when the second wafer is attached to the first wafer, the nozzle and a sealing ring around same are located within the path travelled by the membrane during deflection. Deflections of the membrane change the cross-sectional area of the fluid communication path between the input port and the output port of the fluid manifold. If the deflection is large enough, the membrane seats completely on the sealing ring around the nozzle and completely cuts off flow through the nozzle.
A problem with the above described valve of U.S. Pat. Nos. 4,824,073 and 4,821,997 is that the fabrication process, including the encapsulation of a volume of fluid within a sealed enclosure involves a large number of process steps. It would thus be generally desirable to provide a micro control valve for use in a wide variety of applications including those described for the micro valve of U.S. Pat. Nos. 4,824,073 and 4,821,997 but which is more easily fabricated and less expensive to produce than the micro valves described in those patents.